The purchasing process is pretty easy. Simply select the theme you'd like to purchase, click "Buy" and it should walk you through the rest of the process. If you have trouble, you can contact sales@ithemes.com and they can help you with your purchase.

Customizing your theme is certainly doable, but in order to help you, you'd need to have purchased and installed the theme on your site. Then we'd need to know exactly what customizations you were making. Once you've purchased and installed your Builder Child Theme, you can go to the Builder forumand they'll help you out.